If you've been deep cycling the new battery between 0% and 25% charge a few times, it might have severely diminished capacity already.
If your charger(s) only supply a small fraction of the battery's capacity, can you put the battery in a running car and go for a drive at 30-40A for an hour or so?
A new or nearly new battery, fully charged, should have an at rest voltage of 12.8 or better. (after it has been off the charger for a couple of hours)
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